The present invention relates to a constant pressure delivery valve for a fuel injection pump in an internal combustion engine.
Known in the prior art for use with a high pressure, injection type fuel injection pump is a constant pressure delivery valve in which a pressure wave produced when a fuel injection valve is closed is attenuated to prevent the occurence of a secondary injection. Such a valve is disclosed, for example in Japanese Utility Model Application, Laid-Open No. 63358/83. This constant pressure type delivery valve system includes a spacer block 3 and a valve holder 5, as shown in FIG. 3. The spacer block 3 is fitted into a large-diameter cylindrical portion 25 on the upper end side of a plunger barrel 2 forming a plunger chamber 12, and the valve holder 5 is supported on a surface 4 of the block 3. A flange 24 integrally formed on an intermediate portion of the valve holder 5 is connected by bolts (not shown) to an injection pump body 26 into which the plunger barrel 2 is fitted. Formed in the spacer 3 and the valve holder 5 are discharge passages 22, 6 connected between the plunger chamber 12 and a delivery port 7 in the valve holder 5, and return passages 8, 23 connected between the delivery port 7 and the plunger chamber 12. The delivery port 7 is connected to a fuel injection valve (not shown) mounted on a cylinder head of an internal combustion engine (not shown) through a fuel injection pipe (not shown).
A check valve 10 is inserted in and connected to the discharge passage 22, and an orifice 9 and a check valve 40 are provided in and connected to the return passage 8 in the valve holder 5. The check valves 10 and 40, slightly different in dimension but similarly constructed, are inserted to provide opposite directions of fluid flow. Included in each check valve 10 and 40 is a spring holder 18, a valve body 14 having a ball 13 coupled thereto, and a spring 15 for biasing the ball 13 toward a closed position.
A description will be made only of the check valve 40 with the understanding that the description applies also to the check valve 10. A large diameter cylindrical portion 28 forms a chamber opening into the return passage 23, the cylindrical portion 28 being formed with a threaded hole 19. The spring holder 18 has an externally threaded portion 19 engaged with the tapped hole 19. As shown in FIG. 4, the spring holder 18 has a small diameter, cylindrical shaft portion 18b extending from a shoulder 32 that supports one end of the spring 15. Formed in the holder 18 is a hexagonal tool hole 20 communicating with the return passage 23, a central passage 33 extending in an axial direction of the spring 15 and communicating radial passages 17. Accordingly, the chamber formed by the cylindrical portion 28 is connected to the return passage 23 of the spacer 3 through the passages 17 and 33 and the tool hole 20.
The valve body 14 is provided with a shoulder 35 for supporting an opposite end of the spring 15. Firmly pressed into a semi-spherical depression 31 in an end of the valve body 14 is the ball 13. An edge portion 31a between the ball 13 and the valve body 14 is integrally caulked. The ball 13 is urged by the force of the spring 15 toward a closed position that closes the return passage 8.
When the plunger (not shown) fitted into the plunger barrel 2 is pushed up, fuel in the plunger chamber 12 enters the discharge passage 22 producing a fluid pressure that forces the valve body 14 of the check valve 10 upwardly against the force of the spring 15 into an open position. This allows fuel to enter the cylindrical portion 28 and pass through the passages 17 and 33, the tool hole 20 and the discharge passage 6 in the valve holder 5. The fuel enters a fuel injection valve (not shown) from the delivery port 7 through a fuel injection pipe (not shown) and is injected into a cylinder (not shown). When the plunger moves upward to a level above the effective stroke, pressure in the plunger chamber 12 rapidly lowers, causing the check valve 10 to close and terminate the injection of fuel from the fuel injection valve.
When the plunger subsequently moves downwardly within the range of the effective stroke, pressure in the plunger chamber 12 lowers, and therefore the check valve 40 of the return passage 8 is forced to open against the force of the spring 15. Accordingly, fuel is slowly taken back into the plunger chamber 12 through the orifice 9. This return of fuel is attenuated by the orifice 9 and the check valve 40 to thereby prevent the occurrence of caviation in the fuel injection valve and the check valve 40. Opening movement of the valve body 14 of the check valve 40 against the force of the spring 15 causes the pressure wave from the fuel injection valve to attenuate, thus preventing the secondary injection resulting from reflection of the pressure wave.
The maximum stroke of the valve body 14 is established by the clearance between the closed valve body 14 and the end of the spring holder 18. Upon full opening of the valve body 14 when the abutment surface 14a thereon abuts with the stop surface 18a on the spring holder 18, the return passage 8 remains connected to the passage 23 of the spacer 3 through the cylindrical chamber portion 28, the passages 17 and 33 and the tool hole 20.
In the above described check valve 40, the axial passage 33 connected to the tool hole 20 of the spring holder 18 and the radial passages 17 crossed thereto require expensive machining. In addition, when the valve body 14 strikes the stop surface 18a on the spring holder 18, damage can occur to the small diameter shaft portion 18b of the spring holder 18 that engages and guides the inner surface of the cylindrical, spiral spring 15. The possibility of damage is enhanced by the weakness created in the shaft portion 18b by the radial passages 17. Also, damage to the spring 15 can result from the undesirable friction created between the open edge portions of the radial passages 17 and the spring 15.
The object of the present invention, therefore, is to provide a less expensive and more sturdy constant pressure delivery valve for a fuel injection pump.